Fluid catalytic cracking has been widely adopted for cracking higher boiling petroleum fractions. This process utilizes a finely-divided solid catalyst which is conveyed between the cracking zone, and a separate zone where it is regenerated by controlled oxidation of carbonaceous deposits formed on the surface of the catalyst particles during the cracking reaction.
In early fluid catalytic cracking plants naturally occurring clay catalysts were used. Physical breakdown of such catalysts occurred at temperatures above approximately 1200.degree. F. Regeneration at higher temperatures was known to have certain advantages but was limited by the temperature at which the catalyst began to physically fragment, sometimes referred to as cindering. Later, synthetically produced catalysts replaced the natural clay catalysts because they enhanced the yield, and produced more desirable product mixtures. With the further development of synthetic catalysts, particularly the molecular sieve type, it became possible to operate at higher temperatures without cindering.
Development of fluid catalytic cracking was also limited by the yield of carbonaceous deposits, sometimes called coke, being greater than that required to supply the necessary heat to sustain the process in a balanced mode. Various approaches were developed to regenerate catalyst, while maintaining a heat balance with the cracking process. Schemes such as installing steam coils within the regeneration zone, and circulation of fluidized catalyst through shell and tube heat exchangers were utilized to remove heat from the regeneration zone in an effort to maintain a steady state condition with respect to coke buildup on the catalyst. It developed that catalyst regeneration usually was accomplished by a partial oxidation in which coke was converted to a mixture of water, carbon dioxide, and carbon monoxide.
Because of the catalyst temperature and coke yield limitations, complete oxidation of the carbonaceous deposits to CO.sub.2 and water was avoided. With the development of molecular sieve catalysts having higher thermal stability and producing decreased or controllable coke yields, regeneration by complete oxidation of the carbonaceous deposits became feasible. Various attempts have been made to operate the regeneration zone so as to oxidize the carbonaceous deposits such that carbon monoxide concentration in the flue gas is limited to 1500 ppm by volume or less. Promoters have been introduced into the regeneration zone to increase the oxidation reaction rate. Processes and apparatus also have been designed to maintain the catalyst in contact with the oxidation gases during complete oxidation to low carbon monoxide levels so as to absorb as much of the liberated heat as possible.
Kubizek, in U.S. Pat. No. 2,387,798, discloses a system in which spent catalyst from a fluid catalytic cracker is separated from the cracker flue gas and entrained in a gas stream containing from 0.1 to 10% oxygen to burn off carbonaceous deposits and regenerate the catalyst. Temperatures are maintained between 900.degree. and 1000.degree. F. by controlling the oxygen supply and by cooling the conduit through which the catalyst particles and oxygen-containing gas are passed.
Payne, in U.S. Pat. No. 3,351,548, discloses a system in which spent catalyst particles from a fluid catalytic cracker are stripped of adsorbed volatile hydrocarbons, passed in heat-exchange relationship with hot regenerated catalyst and mixed with cooled regenerated catalyst. The mixture is then entrained in an air stream and a portion of the carbonaceous deposits on the spent catalyst particles are removed by oxidation under conditions which leave a desired residual quantity from about 0.1 to about 0.5% residual coke on the catalyst particle.
Horecky et al., in U.S. Pat. No. 3,909,392, discloses a fluid cracking catalyst regeneration system in which spent cracking catalyst is introduced into a fluidized bed of catalyst particles at the base of a regenerator, and oxygen-containing gas is passed upwardly through the bed to maintain the bed in fluidized condition. Some of the catalyst particles are lifted into the upper portion of the reactor and then allowed to fall back in counter-current relation to the flow of gases through the regenerator in order to absorb heat from the departing gases.
Gross et al., in U.S. Pat. No. 4,035,284, discloses a system for regenerating fluid cracking catalyst particles in which spent catalyst particles from a catalytic cracker are mixed with hot regenerated catalyst particles and passed to a dense fluidized bed superimposed by an upflowing disperse suspended catalyst phase of limited particle density in which carbonaceous deposits on the catalyst particles are oxidized to carbon dioxide and carbon monoxide.
Such processes and apparatus for regenerating catalyst particles are subject to numerous disadvantages. The size of the regenerator required may be unduly large, thereby increasing both the expense of constructing the system and the amount of energy required to maintain the regeneration system at operating temperatures. Moreover, despite self-laudatory statements in prior art patents, prior art systems are not as energy-efficient as would be desired in that they do not maximize the retention of heat from the regeneration process in the regenerated catalyst particles. Prior art systems also may not assure complete removal of carbonaceous deposits in order to restore the catalyst particles to maximum activity and selectivity. In order to completely regenerate catalyst particles, it is desirable to reduce the concentration of carbonaceous deposits to a level less than 0.1 weight percent, preferably less than 0.05 weight percent. Prior art processes further may effect insufficient oxidation of the carbonaceous deposits and thus produce exhaust flue gases containing unacceptably high proportions of carbon monoxide. Environmental regulations require that the carbon monoxide concentration of flue gas be less than 1500 ppm for existing facilities and less than 500 ppm for new processing units. Thus, despite the extensive activity in the prior art, there remains a need for a more efficient and effective process and apparatus for regenerating fluidizable cracking catalyst particles.